Are Coriolis flowmeters a universal technology?

Just a flowmeter? The capabilities of current designs can put them among the most versatile devices in your plant. Coriolis meters can measure mass flow of liquid and gas, density, temperature, and viscosity. One instrument makes the measurements without additional devices or sampling lines, simplifying measurement systems and reducing life cycle costs.

Jerry Stevens, Endress+Hauser, Inc.

10/15/2010

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Coriolis meters have been around for more than 20 years, and initially were regarded as a complex technology only suitable for use in limited mass flow measurement applications. But much has changed since then. Today, Coriolis meters are not only widely used to measure mass flow of liquid and gas, but also to measure density, temperature, and viscosity. All of these measurements are made with one instrument without the need for additional devices or sampling lines, simplifying measurement systems and reducing life cycle costs.

Using these multi-variable measurements, Coriolis meters are effective in a wide range of applications including custody transfer, API Gravity measurements, and cut analysis.

Does all this make Coriolis meters a truly universal technology for flow and related measurements?

Coriolis basics

Basic Coriolis meter construction consists of a dual or single measuring tube (Figure 1) mounted so that it can vibrate. The measuring system is internally driven or excited to oscillate at its resonant frequency. Electrodynamic sensors coupled to the measuring tube monitor the tube frequency, and the difference of the signal phase shift between the inlet and outlet side.

At zero flow, meaning when the liquid or gas media is at a standstill, there is no linear movement and consequently no forces are transferred from the media to the pipe. At this point, the measuring system is said to be in phase.

Once the media is flowing, the movement induced by the oscillating measuring tube superimposes itself on the linear movement of the flowing media. This causes a non-symmetrical force, the so-called Coriolis force, which causes the measuring tube to twist.

Electrodynamic sensors at the inlet and outlet register the phase difference, which are essentially changes in position over time. With respect to time, this phase difference is directly proportional to the mass flow rate.

Coriolis meters measure mass flow with a typical accuracy of ±0.05% and high linearity over a wide turn-down range, which is better than almost any other flow meter technology. Consequently, Coriolis meters are used in custody transfer, calibration, and many other applications where accurate measurement of mass flow is critical.

More than mass flow

A Coriolis meter operates at its resonant frequency, which is dependent on the total mass of pipe and media. This allows the meter to derive the media’s density from the measured frequency signal. A modern Coriolis meter also has a temperature sensor that determines the actual measuring tube temperature. This is used to compensate for temperature related mechanical effects such as tube expansion, which enhances meter stability under changing process conditions

Viscosity measurements can be derived in a single-tube Coriolis sensor because internal transducers monitor and control the power required to maintain the torsional movement of the measuring tube. The torsional oscillation of the measuring tube creates a shear rate on the fluid media which is a function of fluid viscosity. Because the shear force is used to determine the drive current required within the system, viscosity can be calculated.

Using Coriolis technology for in-line viscosity measurement has several benefits as compared to using a separate viscosity meter, including a reduction in the number of required measurement products, an obstruction-free flow path, and elimination of moving parts associated with rotating insertion or vibration probes. Coriolis meters can also accurately measure viscosity in processes that are time- or shear-dependent (Figure 2).

Because multiple variables are available from a Coriolis meter, other process variables can be derived. This is done by taking into account various properties of the media being measured. For example, concentration (Figure 3) can be calculated from a media-specific table once density is known. Other derived variables include but aren’t limited to:

Standardized density measurement to specific temperature references;

Baume specific gravity measurement in light or heavy scale ranges;

Percent solids content or percent volume and mass derivations;

Brix per ICUMSA for dissolved sucrose;

Plato for the beer or brewing industry;

Balling scales for wine making;

Alcohol proof measurement for distillers per TTB Gauging Handbook;

Percent black liquor in the pulp & paper industry;

API gravity for fluids other than water in the petroleum industry; and

Ethanol, methanol, and oil and water cut analysis.

Multi-variable measurement and calculation of associated process variables often eliminates the need for additional specialty devices, sample points, sample conditioning, laboratory analysis, and process modeling. All of these components and procedures can be replaced by a single Coriolis meter.

Measuring gas flow

A Coriolis meter measures mass gas flow, which the meter can convert to standard flow values, such as standard cubic feet per minute (SCFM). The conversion requires knowledge of the standard or base gas density, which can be found in a lookup table. This value is entered into the Coriolis flowmeter as a conversion value. No pressure or temperature measurements are needed from other sensors, so a Coriolis meter is generally easier to use and often costs less than other types of gas flow meters.

A problem may arise if the composition of the gas changes, because the conversion value also changes. One solution for very critical applications is to use a gas chromatograph to calculate the composition in real time, and then use a flow computer to adjust the output of the Coriolis meter according to the composition.

The natural gas industry is accustomed to changing composition, and some users simply ignore the problem. They assume that the changing composition averages out to a constant value, and use a conversion value based on that composition. Another solution is to use the mass flow value such as lb/min instead of SCFM.

A universal meter?

For many applications, Coriolis meters are recognized as the most accurate flow measurement device available off the shelf, and certainly the most accurate mass flow instrument. By making direct mass flow measurements, a Coriolis meter eliminates the need to compensate for other process variables such as temperature, pressure, density, viscosity, and flow profile. No other flow meter type can provide the ability to measure mass flow, volumetric flow, total flow, density, viscosity, and temperature.

Volumetric flow meters are less expensive than Coriolis meters for many applications, but can only measure flow in volume as opposed to weight. To calculate an estimated weight of the media, a volumetric meter must use user-generated values for density. The resulting mass flow calculation isn’t as accurate as a measurement taken with a mass flow meter, often by a considerable amount.

But Coriolis meters can’t be used everywhere. For example, they can’t always accurately measure two-phase flow, usually a combination of two media such as gas and liquid. Entrained air can present problems, so applications where the flow source may run dry and introduce air into the flow tubes—such as unloading tank cars—can cause older design Coriolis meters to shut down. Most modern advanced Coriolis meters have built-in intelligence and can handle entrained air and similar issues, at least to some extent.

Batch processes used to be another source of measurement difficulty for Coriolis meters. In these processes, the meter starts out empty, receives and measures media, then encounters slugs and bubbles at the end of the batch. Recirculation loops were often required to ensure the meter was always filled with media. Modern Coriolis meters are designed to handle these types of applications, and can generally generate accurate measurements.

Coriolis meters have practical maximum size limitations and are rarely built in pipe sizes larger than 10 to 12 in., but most are smaller. At the opposite end of the spectrum, they are available with pipe sizes of 0.1 in. and smaller for very low flow rates, often below 0.1 lb/min. or 0.01 gpm of water.

Other issues that may affect operation of Coriolis meters include:

Unacceptable pressure drop, which is an issue with many types of flow meters;

High energy input required may conflict with safety requirements in hazardous areas; and

Excessive excitation may lead to fatigue failure of the tubing.

Special care must be taken when installing a Coriolis meter to minimize the effects of external vibration. This ensures stability of meter oscillation, promoting accurate measurement and long term reliability.

Replacing other measurement devices

Even after considering the issues noted above, Coriolis meters are often found to be the best choice in many process measurement applications. Coriolis meters can measure flow in many types of media including, but not limited to, cleaning agents, solvents, fuels, vegetable oils, animal fats, latex, silicon oils, alcohol, fruit solutions, toothpaste, vinegar, ketchup, mayonnaise, gases, liquefied gases, natural gas, and steam.

Prior to the recent economic slowdown, Coriolis meter sales were growing at a rate of 12% per year. This was due to widespread applicability, increasing acceptance by end users, and use in applications formerly the exclusive province of other flow meter types.

For example, Coriolis meters are increasingly used in custody transfer of petroleum liquids, usually replacing positive displacement meters. The American Gas Association has approved Coriolis meters for this use, spurring new installations in this and other critical applications.

Coriolis meters are now often used instead of density and viscosity meters as Coriolis meters provide these measurements along with mass flow and temperature. Using one meter to measure multiple variables saves installation time and can lower initial and life cycle costs.

Mass flow has often been measured using various types of weighing instruments, but this can be problematic, particularly in continuous processes. Although there are methods to continuously measure the weight of media flowing through a process, most are cumbersome and expensive when compared to a Coriolis meter.

Integration of digital communications between the meter and the control system unlocks access to all Coriolis variables and diagnostic information. Digital protocols such as EtherNet/IP allow users to directly interrogate field devices from beyond the control system. Using a standard industrial Ethernet-based protocol reduces integration, training, and hardware costs by ensuring device transparency.

Modern Coriolis meter are a true smart meter technology, enabling proactive maintenance as diagnostics parameters are monitored in real-time to gauge process effects. This is beneficial in many cases, such as after sanitization or to determine process impact on a sensor from coating and buildup. Proactive maintenance can increase uptime, leading to substantial savings.

Improvements have been made in Coriolis meters over the last few decades—lowering costs, improving reliability and increasing accuracy. These improvements have expanded the range of suitable applications, allowing Coriolis meters to replace not only other flow meter types, but also other instruments.